Which of these numbers is prime? ${7,\ 33,\ 49,\ 63,\ 72}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 7 are 1 and 7. The factors of 33 are 1, 3, 11, and 33. The factors of 49 are 1, 7, and 49. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 72 are 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, and 72. Thus, 7 is a prime number.